HR: 08:45h
AN: V51D-04 INVITED     [Abstracts]
TI: Non-Newtonian and Viscoelastic Properties of Lava Flows
AU: * Bagdassarov, N S
EM: nickbagd@geophysik.uni-frankfurt.de
AF: Institut fuer Meteorologie und Geophysik, J. W. Goethe Universitaet-Frankfurt, Feldbergstrasse 47, Frankfurt am Main, 60323 Germany
AB: Lava flow models require an in-depth knowledge of the rheological properties of lava. Previous measurements have shown that, at typical eruption temperatures, lavas are non-Newtonian. The reasons for this include the formation and destruction of crystal networks and bubble deformation during shear. The effects of bubbles are investigated experimentally in this contribution using analogue fluids with bubble concentrations $<$20%. The shear-thinning behaviour of bubbly liquids noted by previous workers is shown to be dependent on the previous shearing history of the fluid. This thixotropic behaviour, which was investigated using a rotational vane viscometer, is caused by delayed bubble deformation and recovery when subjected to changes in shear stress. A rotational vane viscometer and torsional deformation apparatus were used to investigate the rheological properties of bubbly liquids and foams in order to determine a viscoelastic transition. These experiments have shown that the foams tested are viscoelastic power law fluids with a yield strength. Non-Newtonian properties and yield strength of foams are shown to be a probable cause of accelerating flow fragmentation in tube flow experiments on expanding foams. The flow of a bubbly fluid through a narrowing conduit may cause a pulsating regime of a flow due to periodic slip and slip-free boundary conditions near the walls of a conduit. Slip boundary conditions can lead to instability in viscoelastic shear flow causing short wavelength fluctuations at high shear rates. This mechanism may also take place during explosive volcanic eruptions. The frequency and amplitude of oscillation shear affect the structure of lavas which are thixotropic non-Newtonian liquids. The frequency dependent structure of lavas can be identified via frequency hysteresis and time-evolution of internal friction and viscosity. The rheological properties of basaltic lavas from Etna, Hawai'i and Vesuvius have been investigated at temperatures between $\sim500$ and 1150$\deg$C using a small strain oscillatory shear. The viscoelastic response of the lavas was analysed using small forced sinusoidal torques ($<10^{-3}$ N m) at frequencies between 0.002 and 20Hz. A purely viscous regime was only approached during experiments with Hawai'i samples. These experiments indicated that between $\sim$1070 and 1130$\deg$C, strain rate independent viscosities ($>10^9$ Pa s) could be measured at strain rates $<$ $\sim10^{-2}$ to $10^{-1} s^{-1}$. At 800$\deg$C, temporal variations in complex shear modulus and internal friction suggest that, over durations of up to 120h, structural adjustments were occurring within some of the samples. This time-varying behaviour of lava samples may be attributed to the slow closing (healing) of micro-cracks and pore space resulting in the apparent stiffening of lava samples under annealing. Thus, those parts of lava flows that undergoing slow cooling have more elastic properties. Regions which cool faster possess smaller shear moduli and higher internal friction due to thermal micro-cracking and less cohesion between crystal grains and bulk glassy matrix.
DE: 8429 Lava rheology and morphology
DE: 8145 Physics of magma and magma bodies
DE: 5120 Plasticity, diffusion, and creep
DE: 3909 Elasticity and anelasticity
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting